Image coding is a widespread technology for transferring pictures/images over communication channels. Coding of an image in a certain way can be achieved by image compression techniques. When a compressed image is transmitted, a valuable transmission time can be saved, as well as memory resources. A conventional image that can be transmitted consists of an array (a matrix) of N×M pixels, wherein each pixel has a unique value that varies according to the content of the image. For example, if the image has a digital representation in a conventional gray scale, each pixel can have numerical values from 0 (black) to 255 (white).
The main problem of image compression is the degradation in resolution. If the whole image is uniformly compressed, areas that contain less information (less intensity changes between pixels, such as a uniform area) are represented by similar data that is required to represent dense areas that contain more information (greater intensity changes between pixels). Thus, more (and unnecessary) data should be stored and transmitted.
“Programmable multiresolution CMOS active pixel sensor” to R. Panicacci, S. Kemeny, L. Matthies, B. Pain and E. R. Fossum, Proc. SPIE vol.2654 Solid State Sensor Array and CCD Cameras, pp. 72–81, 1996, discloses a CMOS Active Pixel Sensor (APS) that allows the flexibility of placing signal processing circuitry on the imaging focal plane. This APS allows x-y addressability of the array for windows of interest and sparse sampling readout of the array. This multiresolution CMOS APS can perform block (a group of selected pixels) averaging, carried out on-chip, in order to eliminate intensive software processing of the image, which is carried out off-chip. However, the obtained average value is based on full resolution.
“Space-variant non-orthogonal structure CMOS image sensor design”, to Pardo, F., B. Dierickx, and D. Scheffer, IEEE Journal of solid state circuits, vol. 33, no. 6, June 1998 discloses a polar imager, in which the resolution is highest in the central area of the sensor array and decreases towards the peripheral areas (e.g., a CMOS log-polar image sensor). However, using such a polar imager requires external control circuitry to focus the camera on the object of interest.
“Pixel parallel and column parallel architectures and their implementations of on sensor image compression” of Aiziwa K., Hamamoto T., Ohtsuka Y., Hartori M. and Abe M. discloses an image compression technique that uses pixel parallel and column parallel architectures. However, in order to achieve the desired compression, an additional full frame analog memory is required.
“A CMOS image sensor with analog two-dimensional DCT-based compression circuits for One-Chip cameras” of Kawahito, S., M. Yoshida, M. Sasaki, K. Umehara, D. Miyazaki, Y. Tadokoro, K. Murata, S. Doushou, A. Matsuzava, IEEE Journal of solid state circuits, vol. 32, no. 12, December 1997 discloses a CMOS image sensor with analog two-dimensional DCT-based compression circuits for One-Chip cameras.
Still, the disclosed image compression is carried out using conventional techniques, in which the resolution parameter is constant for the entire image, resulting in relatively lower quality.
All the methods described above have not yet provided satisfactory solutions to the problem of on-chip image compression that is carried out in real-time and in a compact way, while obtaining a high quality image.
It is an object of the present invention to provide a method and apparatus for image compression of a high quality picture.
It is another object of the present invention to provide a method and apparatus for image compression that provide enhanced on-chip coding of an image.
It is still another object of the present invention to provide a method and apparatus for image compression that provide adaptive coding of an image, which can be carried out in real-time.
Other objects and advantages of the invention will become apparent as the description proceeds.